docs: def build(self, package_dir, output_style='nested'):
          """Builds the Sass/SCSS files in the specified :attr:`sass_path`.
          It finds :attr:`sass_path` and locates :attr:`css_path`
          as relative to the given ``package_dir``.
  
          :param package_dir: the path of package directory
          :type package_dir: :class:`str`, :class:`basestring`
          :param output_style: an optional coding style of the compiled result.
                               choose one of: ``'nested'`` (default),
                               ``'expanded'``, ``'compact'``, ``'compressed'``
          :type output_style: :class:`str`
          :returns: the set of compiled CSS filenames
          :rtype: :class:`frozenset`
  
          .. versionadded:: 0.6.0
             The ``output_style`` parameter.
  
          """
          sass_path = os.path.join(package_dir, self.sass_path)
          css_path = os.path.join(package_dir, self.css_path)
          css_files = build_directory(
              sass_path, css_path,
              output_style=output_style,
              strip_extension=self.strip_extension,
          ).values()
          return frozenset(
              os.path.join(self.css_path, filename)
              for filename in css_files
          )